As the operation speed becomes fast, recent semiconductor devices are now associated with a problem of malfunction caused by noise generated on a power source system. Consider as an example of semiconductor devices a resin sealed semiconductor device wherein a semiconductor chip is mounted on a bed, one end of leads disposed near at the periphery of the chip are connected to pads on the chip by bonding wires, and other end of the leads are exposed outside of the resin sealed device. In the case of a memory chip, there is formed within the chip an internal circuit such as a memory array and an external circuit such as an output buffer. In the case of a microcomputer chip, there is formed within the chip an internal circuit such as a calculation circuit and a so-called peripheral circuit such as an output buffer. In the case of a chip formed of a so-called multi-bit configuration, there has been associated malfunction of the internal circuit caused by adverse effects of a power source potential change during the operation of the external circuit. There are two types of conventional chips. In one type, as shown in FIG. 1, power is supplied from the same power source pad both to the internal and external circuits. In the other type as shown in FIGS. 2 and 3, power is supplied from different power source pads respectively to the internal and external circuits. However, in both types of conventional chips, a power source potential change at the external circuit propagates to the internal circuit.
This propagation of a power source potential change will be described with reference to FIGS. 1 to 3 in this order.
A power source potential change is likely to occur when a plurality of output buffers switch at the same time. In such a case, a potential change .DELTA.V is given by: EQU .DELTA.V=NL(di/dt) (1)
where N is the number of output buffers, L is a self-inductance of a power source system, i.e., a self-inductance of a power source line along a path from an outer lead, an inner lead, a bonding wire, and to a pad on the semiconductor chip, and di/dt is a current-change rate relative to time.
There is a conventional semiconductor device which has a wiring structure such as shown in FIG. 1. Power source pads 74 are provided on a semiconductor chip 72. One of the power source pads 74 is connected to a bonding point 73 on an inner lead of a power source lead 71 by a bonding wire 75.
Assuming that N=8, L=12 nH, and di/dt=1.times.10.sup.7 (A/sec), it can be understood from the equation (1) that a potential change .DELTA.V=0.96 V appears at the power source pad 74 of the semiconductor device. This potential change may cause malfunction in inputting or reading data.
Taking such potential change into consideration, a recent semiconductor chip has a power source pad for driving output buffers and the like (hereinafter called external circuit power source pad) and another power source pad for driving an internal circuit (hereinafter called internal circuit power source pad), to thereby prevent malfunction of the internal circuit.
The wiring between the semiconductor chip and leads of such a semiconductor device is shown in FIG. 2. The semiconductor chip 82 has an external circuit power source pad 84a and an internal circuit power source pad 84b. A power source lead 81 is provided near at the pads 84a and 84b. At the distal area of the power source lead 81, there are formed two bonding points 83a and 83b. The bonding point 83a is connected to the external circuit power source pad 84a by a bonding wire 85a, and the bonding point 83b is connected to the internal circuit power source pad 84b by a bonding wire 85b.
There is another conventional semiconductor device having a wiring structure such as shown in FIG. 3. The distal area of a power source lead 91 is bifurcated into two inner leads 91a and 91b. A bonding point 93a is provided at the end portion of the inner lead 91a, and a bonding point 93b is provided at the end portion of the inner lead 91b. An external circuit power source pad 94a on the semiconductor chip 92 is connected to the bonding point 93a by a bonding wire 95a. An internal circuit power source pad 94b is connected to the bonding point 93b by a bonding wire 95b.
The potential change .DELTA.V of the device shown in FIG. 2 is 0.8 V from the equation (1). This value is smaller than the value 0.96 of the device shown in FIG. 1. The reason for this is that the self-inductance L of the whole power source system is reduced to 10 nH. The self-inductance L of the power source system of the device with the bifurcated inner leads shown in FIG. 3 is reduced to 9 nH, and hence the potential change .DELTA.V is 0.72 V.
Recent semiconductor devices use an increased number of bits in order to speed up the operation. The number N of output buffers for switching operation increases correspondingly. In order to further speed up the operation, it is necessary to enhance the driving ability of output transistors. As the driving ability is increased, the current change rate (di/dt) also increases. As the values of N and (di/dt) increase, the potential change .DELTA.V will increase as seen from the equation (1), resulting in a high possibility of malfunction. With the above description in view, the conventional devices such as shown in FIGS. 2 and 3 have only a small effect of reducing the potential change .DELTA.V and cannot sufficiently prevent malfunction.